In the foundation drilling industry and in the boring and tunneling industry, it is desired to excavate large diameter shafts (on the order of 36 inches to 84 inches diameter and up) penetrating into very hard rock. In the foundation drilling industry, these shafts are typically filled with reinforced concrete to form foundation piles for buildings, bridges, etc, while in the boring and tunneling industry, these shafts are typically used as access shafts, utility shafts, ventilation shafts, personnel entry shafts or elevator shafts. Often rock augers are used, equipped with tungsten carbide cutting edges. When the rock becomes very hard, the progress of the excavation will virtually stop or reach excavating rates less than 2″ per five minute interval with full downward force and with full torque applied to the rock auger.
Alternatively for very hard rock, so-called drilled shaft construction techniques are typically employed, in which a hollow core barrel is rotated so that cutters on its lower edge cut an annular kerf in the rock. Once this kerf is drilled to the desired depth by the core barrel's cutting face, the rock core within the kerf may be broken up and augered out, or broken off and removed. 
The foregoing cutting techniques generally require extreme pressure exerted against the core barrel by the drive mechanism, and removal of the core can be very difficult. For applications which only require smaller-diameter shafts (i.e., less than about 34 inches), it is known to use pneumatic, percussive-type downhole drills, which permit significant reductions in the amount of pressure that must be applied to the drilling apparatus. These relatively small downhole “hammer” drills typically employ a drill bit with a circular cutting face having numerous protruding tungsten carbide buttons. A rotary head or kelly-bar drive causes the drill string to rotate in the shaft, and drilling pipes conduct compressed air to a piston (i.e., the hammer) near the end of the drill string, generating percussive blows of the cutting face of the drill bit to the earth at the distal end of the shaft. These percussive blows place the rock in compression, and the retreating drill bit places the rock in tension. This cyclic action, which may occur several hundred times per minute, breaks up the rock, which is then removed by a drilling fluid (often, simply air) which is circulated into the shaft under pressure. Rotation of the drill string brings the drill bit into contact with fresh unbroken rock during successive percussion cycles.
Single downhole drills of the type described are typically from a few inches up to about 34 inches in diameter and excavate the shaft relatively fast. Greater diameters are impractical due to the excessive cost of larger-diameter drill bits, expensive large downhole hammers and increased compressed air requirements. To achieve larger-diameter shafts, it is known to use cluster drills comprising a plurality of hammer drills in  a gang construction, as described in U.S. Pat. No. 4,729,439 to Kurt. In gang drills of this type, several hammer drills are arranged within a casing in a ring around a central hammer drill which is concentric with the casing and thus the shaft to be drilled. The cutting faces of the drill bits must be sufficiently large to cut swaths which completely cover the distal end of the shaft. For relatively large diameter shafts, e.g., 36 inches and greater, the number and size of hammer drills required make their use impractical because air and fuel consumption tends to be quite high.
In addition, gang drills suffer from disadvantages such as high cost and high maintenance, with attendant high out-of-service times. Also, gang drills lose efficiency when excavating on sloped or uneven ground. All the hammer bits that are not in contact with the ground at a given time will blow off air and severely impair the hammering ability of the hammer bits that are in contact with the rock. Also, the smaller diameter shanks tend to break off when subjected to side loads during rotation of the barrel, resulting in bit replacement and possible expensive retrieval operations.
Moreover, none of the foregoing prior art tools can drill shafts of different diameters, and thus they are unsuited to drilling shaft portions into which casing is to be placed before further drilling takes place. Also, in a vertical or near-vertical shaft, the foregoing drills can not carry cuttings to the surface without adding a calix basket or other catchment to the top of the tool for carrying out cuttings that are not blown out of the shaft. This makes the overall height of the tool so tall as to interfere with the underside of the rotary table on conventional foundation drill machines, making it  difficult to clear the tool from the excavation to dump the cuttings, remove the tool, or inspect the tool.